Artificial leather

ABSTRACT

ARTIFICIAL LEATHER CONSISTING OF (1) A BASE MATERIAL LAYER IN THE FORM OF A FLEXIBLE SHEET OF AT LEAST ONE OF NATURAL FIBERS ARTIFICIAL FIBERS, SYNTHETIC FIBERS, INORGANIC FIBERS AND RESINOUS SHEETS, AND (2) A SUPERPOSED SURFACE LAYER OF AT LEAST ONE LOWER ALKYL ESTER POLYMER OF N-CARBOXYGLUTAMIC ACID. THE ARTIFICIAL LEATHER HAS EXCELLENT MECHANICAL PROPERTIES, PLIABILITY, RESISTANCE TO BLOCKING, RESISTANCE TO HEAT, MOISTURE PERMEABILITY AND RESISTANCE TO WASHING AND ALSO DESIRABLE AESTHETIC PROPERTIES COMPARABLE TO THOSE OF NATURAL LEATHER, SUCH AS APPEARANCE AND FEEL.

United States Patent 3,582,397 ARTIFICIAL LEATHER Kei Shibata, Imiya-shi, Japan, assignor to Ajiuomoto Co. Inc., Tokyo, Japan No Drawing. Continuation-impart of application Ser. No. 471,398, July 12, 1965. This application Sept. 16, 1969, Ser. No. 858,530

Claims priority, application Japan, Aug. 12, 1964, 39/ 46,637 Int. Cl. D06n 3/04, 3/12 US. Cl. 117-1383 1 Claim ABSTRACT OF THE DISCLOSURE This application is a continuation-in-part application of Ser. No. 471,398 filed July 12, 1965, now abandoned.

This invention relates to artificial leather having excellent properties such as mechanical properties, pliability, resistance to blocking, resistance to heat, moisture permeability and resistance to washing and also desirabl aesthetic properties comparable to those of natural leather, such as appearance and feel. More particularly, it relates to artificial leather consisting essentially of (1) a base material layer in the form of a flexible sheet comprising at least one member selected from the group consisting of natural fibers, artificial fibers, synthetic fibers, inorganic fibers and resinous sheets, and (2) a superposed surface layer formed of at least one of N-carboxyglutamic acid lower aliphatic alkyl ester polymers of the formula wherein R is a lower aliphatic alkyl group, e.g. methyl, ethyl, propyl, butyl, pentyl, hexyl, and m is an integer of from 300 to 3500.

Many attempts have heretofore been made to produce artificial leather having a superposed surface layer in the form of a film, sheet or spongy sheet by applying a polymer such as polyvinyl chloride, polyurethane and polyamide to a base material layer of a woven, knitted or nonwoven fabric made of natural or synthetic fibers utilizing such means as coating, impregnation and extrusion. It is well known on the other hand that an attempt to improve one or more properties desirable in the end uses of artificial leather results in the deterioration of one or more other desirable properties, and this constitutes a great set back against the end uses of artificial leather. For eX- ample, an attempt to obtain artificial leather having good mechanical properties results in a product having unsatisfactory resistance to hot water, moisture permeability, and feel. Or an attempt to impart a desirable feel possessed by natural leather results in a product having unsatisfactory mechanical properties and resistance to hot water.

It has now been found that artificial leather having a superposed surface layer formed of the polymer of N-carboxyglutamic acid lower aliphatic alkyl ester expressed by the foregoing formula has desirable appearance and feel comparable to that of natural leather and also excellent 'Ice,

physical properties, such as mechanical properties, resistance to blocking, resistance to heat, moisture permeability, resistance to washing and resistance to weathering, while retaining the uniform quality and freely adjustable thickness and size which are inherent advantages of artificial leather over natural leather.

It was thought that in the art of artificial leather, there would be no room for improvement of a polymer or a polymer composition which forms a superposed layer on a base material layer for artificial leather. For this reason, attempts are presently being made to provide a physical structure resembling that of natural leather, and no consideration has yet been given to the use of the abovementioned polymer of N-carboxyglutamic acid ester as a superposed layer of artificial leather. It has now been discovered that natural leather is made up of collagen protein, and research work has been continued on the utilization of polyamino acids containing a-amino acid as recurring units similar to protein. As a result, it has been found that the use of the specific polyamino acid having the above-mentioned formula gives artificial leather having the foregoing advantages.

It is known that poly-L-glutamic acid partly esterified with lower alkanols, when used as synthetic surgical sutures (a difierent subject matter from that of the present invention) is readily absorbable by body tissues after having performed its task, and that the sutures consisting of the partially esterified product may contain a surface coating formed by using a dilute solution, for instance a 3% solution, of poly-'y-ethyl-L glutamate (see US. Pat. 3,- 371,069). In view of this, it is quite surprising that in the art of artificial leather, whose subject matter is quite different from that of surgical sutures, artificial leather having the various properties mentioned above could be provided by using the specific alkyl ester of the polyamino acid having the foregoing formula. It is surprising in that such specific alkyl esters meetthe requirement of ready absorption by body tissues in the form of the surface coating mentioned above, and therefore the artificial leather of this invention is quite an unexpected result if there are considered the many severe conditions under which artificial leather is used.

Incidentally, a homopolymer of glutamic acid or its partially esterified products have proved unsuitable for the achievement of the objects of the persent invention.

Accordingly, an object of the present invention is to provide artificial leather having desirable appearance and feel comparable to that of natural leather, and excellent physical properties such as mechanical properties, resistance to blocking, resistance to heat, moisture permeability, resistance to washing, and resistance to weathering, while at the same time retaining the uniformity of quality and the freely adjustable thickness and size which are inherent advantages of artificial leather as compared to natural leather. Other objects and advantages of-the invention will become apparent from the description which follows.

The polymer which forms a superposed surface layer of artificial leather according to the invention is expressed by the following formula in which -R is a lower aliphatic alkyl groups, preferably an alkyl group having 1 to 4 carbon atoms, more preferably a methyl group, and m, representing the degree of polymerization, is an integer ranging from 300 to 3500.

The polymers having a degree of polymerization outside the above-specified range are unsuitable for use in this invention. Polymers with a degree of polymerization below the said range result in brittleness of the surface of the final artificial leather product, while those with a degree of polymerization above the said range will be fibrillated by forces such as bending and crumpling.

Poly-(glutamic acid) or polymers obtained by its partial esterification with lower alkanols become dissolved or swollen at a pH of more than about 7, and therefore are useless in the artificial leather of this invention. If a superposed surface layer is formed of those polymers, the resulting artificial leather has poor resistance to washing with such materials as water and soap water under ordinary conritions which leather undergoes. In addition, such leather has poor pliability and poor resistance to weathering. As the solvents for polymers of other amino acids such as poly-L-leucine, poly-L-valine and poly-L- phenylalanine, only a few such as dichloroacetic acid and trifluoroacetic acid, which have a large polarity, are known. But because these solvents attack most of the materials which constitute the base material layer of artificial leather and corrode the metals of the apparatus used for producing artificial leather, they are substantially useless. Poly-fi-benzyl aspartate is poor in resistance to heat, and blocking tends to occur in the final product. In short, these polymers, when used to form the superposed surface layer, give unsatisfactory artificial leather products.

The choice of base materials in the artificial leather of the present invention is not critical so long as it is not contrary to the technical common knowledge in the art of artificial leather. The base material layer in the form of a flexible sheet in the final artificial leather product of the present invention may comprise at least one member selected from the group consisting of natural fibers such as cotton, ramie, wool, silk and pulp, artificial fibers such as viscose rayon, acetate rayon, regenerated collagen fiber and protein fibers, synthetic fibers such as polyvinyl chloride, acrylic polymer, polyester, polyamide, polyglutamate, polyurethane, polyurea and polyolefin fibers, inorganic fibers such as glassifiber and rock wool, and resinous sheets such as those made from polyvinyl chloride, acrylic polymers, polyamides, polyglutamate, polyurethanes, polyolefins and synthetic rubbers. Accordingly, it is possible for instance, to use a base material layer comprising two sublayers, one being of a fibrous structure consisting of a woven, knitted or non-woven fabric of these fibers and the other, of a resinous sheet made from such a material as polyurethane, polyvinyl chloride, nylon or neoprene rubber. These polymers include both homoand copolymers.

Specific examples of the base material layers usable in the present invention are those having a polyurethane sheet on a cloth which are prepared by coating a solution of polyurethane in an organic solvent such as toluene on one surface of a cloth such as a knitted cloth, woven cloth or non-woven cloth of cotton or a blend of cotton with other fiber which surface has been subjected to gigging, or by bonding a polyurethane sheet to the surface of the cloth by means of an adhesive, or by forming a polyurethane sheet on a release paper and transferring it to the napraised surface of the cloth while the polyurethane is still in an uncured state, followed by stripping off the release paper; those comprising a polyvinyl chloride sheet, polyamide sheet or neoprene rubber sheet heat-bonded onto a cloth; those prepared by coating a cloth with a paste or a solution in an organic solvent of polyvinyl chloride, a solution of nylon in an organic solvent or a solution of synthetic rubber in an organic solvent; and those having a polyvinyl chloride sheet on a cloth prepared by transferring an organic solvent paste of polyvinyl chloride in accordance with the same procedures as in the case of the above-mentioned polyurethane.

The superposed surface layer of the specific polyglutamate of the invention which contributes to the attainment of the many advantages of artificial leather may be provided, for instance, by any of the following procedures: by coating a solution of the polyglutamate in an organic solvent; by forming a film or sheet of the polyglutamate according to the casting process and bonding it by means of an adhesive;- by laminating the film or sheet on a base material comprising a polyurethane sheet on a cloth while the polyurethane sheet is still in an uncured state, under pressure or both under pressure and under heat, followed by curing; or by forming a layer of the polyglutamate on a release paper, forming a layer of polyurethane thereon, transferring them to a cotton cloth while the polyurethane layer is still in an uncured state, and bonding them to each other under pressure or under both pressure and heat whereby a part of base material layer and the superposed surface layer are formed at the same time. Depending upon the physical shape of the base material, and/or the'procedure for formation of the superposed surface layer, said superposed surface layer may partly penetrate into the interior of the base material, or it may be present only on the surface of the base material, e.g. in the form of a film.

As examples of the solvent for the polyglutamates expressed by the foregoing formula, there can be mentioned halogenated hydrocarbons such as chloroform, methylene chloride, dichloroethane, trichloroethane, and tetrachloroethane, aromatic hydrocarbons such as toluene and xylene, acetate such as methyl acetate and ethyl acetate, ketones such as acetone, ethyl methyl ketone and methyl isobutyl ketone, dimethyl formamide, dimethyl acetamide, methyl pyrrolidone, 'y-butyl lactone, and combinations of these with one another. The concentration of the polyglutamate in the solvent solution depends upon such factors as the type and polymerization degree of the polyglutamate and the solvent used, but usually concen trations not in excess of by weight, particularly 20% by weight or less are preferable. The use of the polyglutamate at too high a concentration is not desirable because it tends to be oriented to a relatively large degree by flowing. If the polyglutamate has a lower degree of polymerization, higher concentrations may be used, but if it is high, the lowest possible viscosities should preferably be selected.

The polymer having the foregoing formula which forms a superposed surface layer in the present invention may contain additives usually used in artificial leather, such as coloring agents (dyestuffs and pigments), delustering agents andfoaming agents. If desired, it may further contain a minor amount, at most by weight, usually not more than 20 parts by weight (based on the polymer of the above formula) of other polymers capa ble of being blended therewith. It is also possible to utilize polymers of alkyl glutamates which contains two or more different alkyl groups represented by R in the foregoing formula.

The formation of a superposed surface layer of the polyglutamate may be effected in any manner usually practiced in the manufacture of artificial leather, for instance, by a dry process in which the polyglutamate is coated or impregnated by means of a reverse roll coater, nip roll coater, gravure coater, dip coater, kiss roll coater, floating knife coater, blanket knife coater, knife over roll coater, cushion knife coater, air knife coater, inverted knife coater, wire bar coater, brush coater, or sprayer, and the solvent is evaporated, or by a wet pocess in which the polyglutamate is immersed in a non-solvent, coagulated and dried, or by a combination of these processes. It is of course possible to finish the surface of the artificial leather after the formation of the superposed surface layer by embossing it with an embossing roll, calendering it by a calender roll or roughening the surface with sand blast.

With a viewto demonstrating that the artificial leather of the invention having a surface layer of the polyglutamate has far more excellent properties than the conventional artificial leathers, six specimens of artificial leathers for use in clothing were produced in the same manner, shown in Table 1 below. The table also shows the propexcept using a difierent coating material for each artierties of natural leather (kid), and also those of sample ficial leather, and their properties were compared with (6) which is prepared by forming a spongy layer of nylon one another. on a twill fabric of the same thickness produced from The preparation of the artificial leathers was effected multifilaments of the partially esterified product of (5) as follows:' above, and applying the polyamino acid of (1) above Two hundred grams of a wt. percent solution of thereon in the same manner.

TABLE 1 Poly (amino acid) partially Natural Polyglutaesterified leather m 'e Polyurethane Polyamide Polyacrylate product Sample Test items (control) (1) (2) (3) (4) (5) (6) Elongation (percent) Tear strength (kg.mm.)

Rupture strength (kg/cmfl): 17 15 1e 1e 15 Resistance to crumpling. .10,000 or more. 10,000l5,000 s 10,000l5,000 4,5005,500 3,5004,500. 8004,000 400.

Resistance to bending 2001-1300 d 250 thousand 250 thousand" 100 thousand 100 thousand" thousandhfl. 5 thousand.

ousan Resistance to scratch (g) 1 000 1,500.

Color iastness to rubbing (Grade): 3

Benzine 3 Resistance to Weathering (Grade):

Diseoloration 1 Deterioration 1 1 Resistance to blocking 200 C. or 200 C. or 200 C. or more.

more. more. Resistance to heat: 150 C. (dry) Good Good Good Not good Not good"..- Good Good. 120 0. (wet) do do Not good do do t, Not good Good (But the base material broken).

Resistance to cold:

Moisture penneabili Resistance to molds. Appearance and feel.

nylon 6 in a mixed solvent of methanol and CaCl was The test methods for the items listed in Table 1 above coated on a 0.40 mm. thick twill cotton cloth by means are as follows: of a knife coater. The so-treated cloth was immersed in Thickness warm Water effect coagulaton' Caclz 4.0 JIS K6553 11s stands for Japanese Industrial Standeluted in warm water by means of a roll, and the cloth ards) was washed and dried. There was obtained a substrate Tensile strength material COIlSlSlIlIlg of a 0.37 mm. thlck nylon 6 spongy layer and a 0.40 mm. thick base cloth and having a de- 113 3 gree of moisture permeability of 3100 g./m. /24 hrs. 4, Elongation Each of the following five coating agents having a resin- 0 HS K 6553 ous content of 10% by weight was coated thereon by Tear strength means of a reverse roll coater to form a 0.03 mm. thick surface layer. The coating agents each contained 5% by HS K4553 weight, based on the resinous content, of carbon black Rupture Strength as a coloring material. 0 13 6553 (l) Polyglutamate (present invention): Resistance to crumpling (a) Poly-'y-methylglutamate (b) Solvent: dichloroethane (2) Polyurethane:

(a) A polyester-urethane resin (trade name of Sanprene L3) obtained by reaction of a urethane prepolymer, prepared from a polyester glycol and a diisocyanate, with a chain-extending agent such JIS K6772The number of crumpling operations conducted until there is a break in the surface layer.

Resistance to bending The specimen is bended at a rate of 100 times per minute, using a flexometer. The number of bending operations conducted until the occurrence of cracking on the as hydrazine (b). Solvent: a mixture of dimethyl formamide and Surface layer 1s measured' dioxane 1: by volume) Resistance to scratching Poll/amide} The specimen is scratched by means of a rod having zne al 02 32;? 112825 2 5 11 1 3 2 351 1 2 a 0.5 mm. radium and ahemispherical end point, which rod nd f 2;} 1d hyd V1 11 dt 15 held vertically against the surface of the specimen. a or a e V (common Y Ca 6 VP my Resistance to scratching means the lowest load of those trade name TOfeSlIl) at which a streak on the surface is observed.

(b) Solvent: methanol (4) Polyacrylic ester:

(a) A terpolymer consisting essentially of methyl HS L-l043 acrylate and butyl acrylate (trade name Honeylon) Grade 5: No color fading (5) Partially esterified poly(amino acid): Grade 1: Conspicuous fading (a) 70% methyl esterified poly-Lglutamic acid (b) Solvent: trifluoroacetic acid The properties of the obtained artificial leathers are (1) Discoloration: The specimen is exposed for 300 Color fastness to rubbing Resistance to weathering hours in weather-o-meter, and the color difference (AE) thereof is measured by a color-difference meter.

Grade 1: AE 0.5 Grade 2: 0.5 AE52.0

Grade 3: 2 AE (2) Deterioration: The specimen is exposed for 300 hours, and thereafter its strength is measured. The ratio (R) of retaining the initial strength is thus computed.

Grade 1: Rz80% Grade 2: 80% R 60% Grade 3: 60% ER Resistance to blocking The specimen (4" 1.1") is placed under a load of 400 g., and the temperature is raised to a predetermined point and maintained for 5 minutes at this temperature, using a scorch tester.

Resistance to blocking means the lowest temperature of those at which blocking occurs.

Resistance to heat The specimen is ironed by using a l-pound iron, and the irregularity of the surface is examined. Good means that no irregularity occurs on the surface.

Resistance to cold JIS K-6772The occurrence of cracking on the surface is examined. Good means that no cracking occurs on the surface.

Moisture permeability Measured in accordance with JIS Z-0208. It is judged that when the value is above 2000 g./m. /24 hrs., the leather does not get stuffy during use, and the designation good is used. Not good refers to leather having moisture permeability below this value.

Resistance to molds The following five strains are used.

(a) Cladosporium herbarum (b) Chaetomium globosum (c) Aspergillus niger (d) Penicillium citrinum (e) Aspergillus glaucus var. tonophilus Yeast malt broth is put into cultivating tubes. Each of the above specified strains (a) to (e) is inoculated into each of the tubes and grown there. The specimen, in the form of a film having a length of 12 cm., a width of 1.2 cm. and a thickness of 0.3 mm., is put into the tube, and cultivation is effected for 6 months at a temperature of 25 C. Thereafter, the attacking of the specimen by the \molds is observed. Good refers to a state where no appreciable attacking occurs, and not good, to a state where a part of the specimen is lost by dissolution, and its weight is decreased.

Appearance and feel This estimation is made by 20 experts in the leather arts. Each person assigns to each specimen any of marks 4, 3, 2 and 1 in which a larger number indicates a more approximation to natural leather, and the number appearing in each column is the sum of the marks given by 20 experts.

Example 1 One gigged surface of a 0.5 mm. thick cotton-nylon (50/ 50 by weight) blended yarn cloth was coated with a 20 wt. percent solution of nylon '6 in a CH OH-CaCl mixed solvent by a knife coater in an amount of 100 g. mF. The so-treated cloth was immersed in warm Water at 60 C. to effect coagulation, followed by elution of CaCl by squeezing it between rolls in warm water at 60 C. It was again washed with warm water, and dried to form a spongy layer of nylon. The resulting base material was cooled with a 10% solution of poly-'y-rnethylglutamate (degree of polymerization of 1500) in chloroform in an amount of 30 -g./m. by means of a reverse roll coater, followed by drying at to C. to obtain artificial leather. The resulting artificial leather was passed between embossing rolls to give it a natural leather-like embossment.

The obtained artificial leather was pliable, and usable in furniture and clothing. The surface exhibited a feel quite resembling that of natural leather. It also had an excellent moisture permeability, and did not get stuffy during use.

Example 2 A urethane resin composition consisting of 200 parts of polyester glycol (trade name Nipponlan 5025), 12 parts of isocyanate compound (trade name Collonate L), 10 parts of curing agent (trade name NY-2), 2.5 parts of a coloring agent, and 80 parts of ethyl acetate, which had been stirred for 2 hours at room temperature and deformed, was spread by a knife coater on a release paper in an amount of 200 g./m. 'It was then dried at to C. to form a film of the polyurethane resin. A 0.4 mm. thick knitted cloth of a blended yarn of nylon and polynosic fiber (50/ 50), one surface of which had been gigged, was bonded onto the polyurethane film on the release paper, and then aged for 72 hours at 50 C. Thereafter, the release paper was taken away, and a base material for artificial leather was obtained. A 5% solution of .1:1 mixture of poly-'y-methyl glutamate (degree of polymerization of 600) and poly-' -butyl glutamate (degree of polymerization of 2000) in a 1:1 mixture of dichloroethane and methyl ethyl ketone was applied three times to the base material by means of a gravure coater (100 meshes), and then dried to form a 5 micron thick film of said polyglutamate on the surface of the polyurethane resin. The obtained artificial leather was embossed with an embossing roll.

The obtained artificial leather had a good pliability, and was usable in gloves, caps and hats. As the polyglutamate does not turn yellow, the artificial leather can be colored very vividly, and thus shows a beautiful appearance. Furthermore, it is not tacky, and has wearing comfort as a glove which cannot be obtained with other artificial leathers.

Example 3 A 15 wt. percent solution of poly-'y-butyl glutamate (degree of polymerization of 3000) in a toluene/ ethyl acetate mixed solvent was applied to a release paper by means of a knife coater in an amount of 200 g./m. and then dried at 100 C. to form a 25 micron thick film. A paste of polyvinyl chloride resin was applied by means of a roll coater to the poly-' -butyl glutamate, and dried. A 0.60 mm. thick cotton cloth, one surface of which had been gigged, was bonded onto the above-mentioned film through the paste. The release paper was then taken away, and the bonded material was passed through a foaming oven for 2 minutes at 220 C. Subsequent embossing and heat bonding under pressure gave artificial leather.

The obtained artificial leather has a warmth retaining effect when used as winter clothes, and has a very good wearing comfort with its excellent moisture permeability.

Example 4 A needle-punched, 1.5 mm. thick non-woven fabric consisting of 2 inch long polyester staples was immersed into a 20 wt. percent dimethyl formamide solution of poly-'y-butyl glutamate (degree of polymerization of 1000) containing 50% by weight of polyurethane (trade name CRISBON No. 6666). Excess solution was removed solution of poly-'y-methylglutamate (degree of polymeriza- H) tion of 1000) in a 7:3 mixture of 1,1,2-trichloroethane and methyl acetate by means of a knife coater, and dried to give artificial leather having a thicknes of 1.1 mm. The obtained artificial leather was used in making shoes. The

surface thereof was beautiful and resembled natural leather. It was also good in resistance to scratching. Furthermore, it had a good moisture permeability, and did not get stuffy during use.

Example 5 N-carboxyanhydride of 'y-methyl glutamate was dissolved in dichloroethane to form a wt. percent solution, which was then polymerized. The resulting solution was coated onto paper of high aeration such as Japanese paper,

and the solvent evaporated at a temperature below 100 C. Thus a film was formed on the paper. The properties of the resulting artificial leather product utilized as a book cloth material are shown in the table below.

Iclaim:

1. Artificial leather exhibiting improved moisture permeability and resistance to crumpling, bending, weathering and scratching, consisting essentialy of (1) a base material layer in the form of a flexible sheet comprising at least one of (a) natural fibers, (b) artificial fibers, (c) inorganic fibers, (d) synthetic fibers derived from an organic synthetic polymer selected from the .group consisting of polyvinyl chloride, acrylic polymers, polyesters, polyamides, polyurethane, polyurea and polyolefins, or (e) resinous sheets derived from an organic synthetic polymer selected from the group consisting of polyvinyl chloride, acrylic polymers, polyesters, polyarnides, polyurethane, polyurea, polyolefins and synthetic rubbers, and

(2) a superposed coating consisting essentially of at least one substantially completely esterified lower alkyl ester polymer of N-carboxyglutarnic acid of the formula L t L.

wherein TABLE 2 Test pp Properties examined Results 3 kgJOmfi load applied at 200 C for 5 hours Heat resistance NO deterioration. Tested with a bending system coldproof tester Coldproofness No deter orat on at C. 30,000 abrasions under a load of one kg./cm. Superficial abrasion resistance No deterloration. Fade-O-meter; 300 hours Lightprooiness D0. At 0. temperature for 30 minutes Dimensional stability About 1% shrinkage. Immersed at normal temperatures for 30 minutes in Resistance to solvents Not spoiled by gasoline; slightlyswoilen.

gasoline, methanol and trichrene.

with methanol; spoiled by tric-hrene.

References Cited UNITED STATES PATENTS 2,302,167 11/ 1942 Austin. 2,630,423 3/1953 MacDonald et a1. 2,636,877 4/ 1953 Coleman. 2,657,972 1 1/ 3 Woodward. 2,728,737 12/ 1955 Wittcofi. 2,729,621 1/ 1956 Miegel. 3,004,004 10/1961 Fox. 3,035,943 5/ 1962 Nottebohm et a1. 3,052,655 9/1962 Fox et a1. 3,076,790 2/ 1963 Fox et a1. 3,371,069 2/ 1968 Miyamae et al.

FOREIGN PATENTS 967,451 3/ 1950 France. 15,792 10/1962 Japan.

WILLIAM D. MARTIN, Primary Examiner J. E. MILLER, JR., Assistant Examiner US. Cl. X.R. 

